Scott-
You seem to have missed the basis for the thinning. Underbrush and small trees are a greater fire risk - more likely to catch fire, more likely to burn & kill the tree, shrub, etc. They aren't economical to harvest though. Larger trees are more likely to survive a blaze. And are economical to harvest.
If you log the trees more likely to be living after a fire, and keep the kindling, how is this making the forest less likely to catch fire?